Optical system and method of designing optical system

Abstract

An optical system includes a diffractive optical element and a refractive optical element. The diffractive optical element corrects a chromatic aberration flare component of wavelengths other than a predetermined wavelength (a design wavelength) remaining in the refractive optical element by means of an aspherical component given by the grating pitch of a diffraction grating included in a diffractive portion of the diffractive optical element. In addition, a refractive portion of the diffractive optical element is adapted to correct an aberration component as a composite of the aberration component of the design standard wavelength shifted by the correction and the aberration component of the design standard wavelength of the refractive optical element.

1. An optical system comprising:
 a refractive optical element; and 
 a diffractive optical element, 
 wherein a diffractive portion of said diffraction optical element corrects a chromatic aberration flare component of wavelengths other than a predetermined wavelength remaining in said refractive optical element by means of an aspherical component given by a grating pitch of a diffraction grating that constitutes said diffractive portion, and a refractive portion of said diffractive optical element has a shape so as to correct an aberration component as a composite of an aberration component of said predetermined wavelength of said diffractive optical element shifted by the aforementioned correction and an aberration component of said predetermined wavelength of said refractive optical element. 
 
   
   
     2. An optical system according to  claim 1 , wherein said diffractive optical element has a refractive portion having an aspherical surface shape. 
   
   
     3. An optical system according to  claim 1  that satisfies the following conditional expression:
   |(λ0/2π)·(φ( Y )−φ0( Y ))|<|( N′−N )·( X ( Y ) − X 0( Y ))|; 
   and Y≠0, 
 
     where,
 φ(Y): phase shape of the diffractive portion represented by the following polynomial expression:
   φ0( Y )=(2π/λ0)( C   1   Y   2   +C   2   Y   4   +C   3   Y   6 + . . . ), 
 
 φ0(Y): phase shape corresponding to the optical power given by the diffractive portion represented by the following formula:
   φ0( Y )=(2π/λ0) C   1   Y   2 , 
 
 Y: height along a direction perpendicular to the optical axis, 
 λ0: design wavelength, 
 Ck: phase factor (k=1, 2, 3, . . . ), 
 X(Y): said aspherical surface shape representing the shift amount in the optical axis direction with the reference at the vertex on the optical axis, at height Y along a direction perpendicular to the optical axis, 
 X0(Y): a spherical surface shape having a curvature equal to the paraxial curvature of said aspherical surface shape, representing the shift amount in the optical axis direction with the reference at the vertex on the optical axis, at height Y along a direction perpendicular to the optical axis, 
 N′: the refractive index, at said predetermined wavelength, of the medium just after (with respect to the traveling direction of rays) said refractive of the aspherical surface shape, 
 N: the refractive index, at said predetermined wavelength, of the medium just before (with respect to the traveling direction of rays) said refractive portion of the aspherical surface shape. 
 
   
   
     4. An optical system according to  claim 1 , wherein said diffractive optical element comprises a plurality of diffraction gratings including at least two materials, and the maximum optical path length difference of rays passing through the diffractive portion of said diffractive optical element is an integral multiple of a plurality of wavelengths. 
   
   
     5. A method of designing an optical system having a refractive optical element and a diffractive optical element comprising:
 a first step to correct a chromatic aberration flare component of wavelengths other than a predetermined wavelength remaining in said refractive optical element by means of an aspherical component given by a grating pitch of a diffraction grating that constitutes a diffractive portion of said diffractive optical element; and 
 a second step to correct an aberration component as a composite of an aberration component of said predetermined wavelength shifted by the correction of said first step and an aberration component of said predetermined wavelength of said refractive optical element by means of a refractive portion of said diffractive optical element.